Temperature gradient furnace apparatus, and method of forming same



n LR AME TPAMAS EAS% E G1 .C RAM JNM R O sURl HFMV. TT a UNFM AEod T.. HDD@ A01 .RWM EGE .EM WR May 4, 1965 United States Patent C) 3,181,847 lTEMPERATURE GRADIENT FURNACE APPA- RATUS, AND METHOD F FGRMING SAME Wiliard E. Hauth, Jr., Flint, and Arthur V. Somers, Fins-hing, Mich., assignors to General Motors Corporation,

Detroit, Mich., a corporation of Delaware Filed May 10, 1963, Ser. No. 279,569 4 Claims. (Cl. 263-41) This invention relates to industrial furnaces and more particularly to a furnace of the temperature gradient type which is particularly useful for subjecting a plurality of ceramic metallurgical samples and the like simultaneously to a plurality of relatively closely related temperatures.

The art of making ceramic articles, for example, from new ceramic compositions usually requires that a series of samples be subjected to a series of closely related tiring temperatures to find the optimum tiring temperature. This may, of course, be done by subjecting a series of samples to a series of independent tiring operations in a conventional box-type furnace. However, this is obviously time consuming and expensive. A temperature gradient furnace is capable of heating a series of samples simultaneously to respectively a series of known graduated temperatures. Typical temperature gradient furnaces available commercially are relatively high in initial cost and expensive to maintain especially if they are used at relatively high temperatures up to about 3200 F. Moreover, they are inflexible as to use in that they are useful essentially as a temperature gradient furnace.

It is the basic object of this invention to provide an economical and efficient temperature gradient furnace. It is a further object of this invention to provide a method of furnace construction by which the conventional boxtype furnace may be converted to an eiicient temperature gradient furnace and subsequently reconverted to an ordinary box-type furnace at a small cost in materials and labor. These and other objects are accomplished by positioning a retort in the form of a refractory tube in the furnace which extends horizontally through the door opening and into the heating chamber of an ordinary box-type furnace and bricking the retort in with refractory bricks at the door opening which forms a wall. The Wall irnmediately surrounding the tube is shaped so as to form a conical or funnel-shaped cavity about the tube over a substantial portion of the wall with the large opening thereof facing the furnace heating chamber whereby the retort tube in the conical cavity portion of the wall is subjected to a progressively decreasing amount of heat in the direction of the apex of the cavity. The portion of the tube within the wall is calibrated for a specified furnace temperature by measuring the temperature therein at convenient intervals beginning at the side of the wall to a point within the heating chamber by means of a suitable thermocouple. A temperature gradient curve is then drawn relating the temperature readings to their position within the tube. A suitable refractory boat or tray is provided which may be moved a predetermined distance into the gradient portion of the tube.

Other objects and advantages of the invention will be apparent from the following description thereof, reference being had to the accompanying drawings, in which:

FIGURE 1 is a cross-sectional elevation view of a temperature gradient furnace;

3,181,847J Patented May 4, 1965 FIGURE 2 is an end view of FIGURE l;

FIGURE 3 is an enlarged view of the retort portion of the furnace shown in FIGURE 1; and

FIGURE 4 is a graph showing the graduated temperature within the retort.

Referring to the drawings, FIGURES 1 through 3 show a box-type gas-fired furnace which is in part of more or less standard construction. This part consists essentially of an outer steel shell 10 of rectangular or boxlike configuration and open at one end which is lined on its upper, back and bottom walls with a refractory brick Wall 12 which may be in the neighborhood of approximately a foot in thickness and which encloses the heating chamber 13. The refractory brick may be the conventional re brick composed of silica and alumina plus impurities. For temperatures above about 27.50 F. a brick containing a relatively high content of silica or alumina (sillimanite, mullite or fused corundum) is used. The interior surfaces 14 may include a refractory coating of similar materials as is well known in the art. The ring means (not shown) may be any of the well known types such as direct tiring, overliring, undertiring, side ring and the like.

A furnace made in accordance with this invention by converting the box-type construction above-described in terms of a specific example involves the provision of a refractory alumina tube 15 preferably closed at one end 15 which is partially inserted into the heating chamber 13 and bricked in at the door opening of the box-type furnace by means of the refractory brick wall 16. This tube in the instapt illustration is three inches in diameter and thirty inches long. It is supported within the heating chamber by a refractory brick 1S positioned at the base of the heating chamber, and immediately surrounding the tube, the brick or bricks Ztl are shaped so as to form the funnel-shaped cavity 22, the large opening of which is located at the interior surface 23 of the brick wall 16 and is exposed to the heating chamber 13. In the instant specific embodiment described, the tube 15 projects a distance A of thirteen inches into the heating chamber 13. An intermediate portion B of the tube which extends through the wall 16 is nine inches in length which, of course, is also the thickness of the wall. The portion C extending outside the furnace is eight inches in length.

It may readily be seen that the diameter and the length of the funnel-shaped cavity 22 will effect the degree of heat transfer to the tube along its entire length within the cavity 22 and will create a temperature gradient within the portion B of the tube located in the wall 16. In contrast, if the tube is bricked into the wall without this funnel-shaped cavity but with the bricks juxtaposed to the tube throughout the entire wall thickness 16, there will be a sharp temperature drop at the wall surface 23 where the tube leaves the heating chamber 13 and enters the brick wall 16. It will also be apparent that a variation in the dimensions of the conical cavity V22 will influence the character of the temperature gradient produced in the tube in this area. A relatively small portion of the wall 16 adjacent the outer surface 28 is bricked in in juxtaposition to the tube 15 to provide for suitable heat insulation. The bricks in the wall 16 may be simply laid up without mortar once the desired shape of the cavity 22 is determined and hence may be readily removed and replaced by other suitable bricks whereby the furnace may be alternately used as a temperature gradient furnace or as a conventional furnace.

In the use of the furnace, the samples to be heated are positioned within the tube 15 along a two-inch wide and 13-inc'n long refractory boat or tray 25 inserted within the tube. This boat merely is slid into the three-inch tube and positioned horizontally so that the outer end of the tray is aligned vertically with the outer surface 2S of the brick wall 16 surrounding the tube. This may be accomplished by employing a rule or graduated handle (not shown) to push the boat into the tube to a depth of exactly eight inches which is the length of the portion C of the tube. Thus, exactly four inches of the tray extend into the heating chamber 13 of the furnace. This arrangement provides a relatively narrow temperature gradient within the tube in the maximum temperature zone within the chamber 13 which becomes increasingly greater toward the small end of the conical chamber 22 and the outer side 28 of the wall.

In the specific embodiment shown, the cone portion 22 f the wall 16 has a length of iive inches, measured at the surface of the tube 15, and the conical walls of the chamber 22 are at an angle of ten degrees to the longitudinal axis of the tube.

Once the furnace has been brought up to the maximum temperature desired, the temperature gradient is determined by measuring the temperature within the tube at suitable intervals in the temperature gradient portion thereof. A typical temperature gradient'resulting from the cone-shaped cavity 22 described above and shown in FIGURES 1 and 2 is shown in FIGURE 4 when the furnace is heated to maintain a constant temperature of 3200" F. in the chamber 13. i

To this end, a suitable platinum-rhodium thermocouple 24 is inserted into the tube 15. The thermocouple wire is encased in a two-hole alumina insulating tubing, which is marked at one-inch intervals using an ink which will withstand the temperature involved.

Since the position of the boat 26 within the tube is known, it is a simple matter to insert the marked thermocouple 24 and take readings at one-inch intervals at any desired point. Only one thermocouple is needed to determine the temperature gradient. As shown in FIGURES 1 and 3, the thermocouple 24 preferably is inserted into the space between the bottom of the boat 26 and the lower Wall of the tube 15. The closed end 15 of the tube is used as a target for an optical pyrometer whereby the temperature in the chamber 13 may be readily measured and controlled.

The temperature gradient curve shown in FIGURE 4 gives the temperature within the tube at one-inch intervals beginning at the outer surface 28 of the Wall 16 to a point within the chamber 13. The samples on the boat Z5 are located at one-inch intervals, and therefore their location within the tube is known. The temperature to which each sample is subjected may be readily taken from the curve.

It is readily apparent that the length of the cone 22 and the angle formed by the cone wall and the tube may be readily varied to change the temperature gradient. It has been found that using a wall 16 having a thickness of about nine inches, cone lengths of from three to eight inches and angles between the cone Wall and the tube of from tive to fifteen degrees produces morst satisfactory temperature gradients.

Although this invention has been described in terms of a specific example and as applied to a gas-tired box-type furnace, it will be apparent to those skilled in the art that the principles of the invention may be applied to other furnaces and considerable variations may be made in the shape and size of the gradient zone 22 without departing from the spirit of the invention.

We claim:

1. A method of converting a conventional box-type furnace comprising top, bottom and side refractory walls enclosing a heating chamber and having a doorway in one Cil side thereof, the steps comprising forming a refractory brick wall across said doorway having an opening therethrough, said opening having a frusto-conical configuration over at least a portion of its length with the largest diameter thereof being positioned at the inner side of said brick wall, inserting a closed end refractory tube through said opening with said closed end extending into said heating chamber and the smallest diameter portion of said opening snugly engaging said tube to form a heat darn, temperature Calibrating the portion of said tube Within said opening at convenient intervals for a particular operating furnace temperature and providing means within said tube for supporting articles to be heated at said calibrated intervals.

2. A furnace for simultaneously heating a plurality of articles to a plurality of different known temperatures along a temperaure gradient curve comprising refractory walls forming a heating chamber, one of said walls having an opening therethrough, a refractory tube having a closed end extending through said opening with said closed end extending into said enclosure, said opening having a frusto-conical portion with the widest diameter thereof being located adjacent said chamber and the smallest diameter thereof closely surrounding said tube, the space within said frusto-conical portion between said tube and said one wall being free of obstruction and completely exposed to said enclosure, said frusto-conical portion establishing an area Within said tube having a predetermined temperature gradient for a predetermined enclosure temperature, an elongated tray within said tube extending along said frusto-conical portion for supporting articles to be heated at predetermined points along the length thereof, the said space within said tube within said frustoconical portion being temperature calibrated along the length thereof whereby the temperature at each of said points is known for said predetermined enclosure temperature.

3. A furnace for simultaneously heating a plurality of articles to a plurality of different known temperatures along a temperature gradient curve converted from a boxtype furnace comprising top, bottom and side refractory walls forming a heating chamber and having a doorway in one side thereof, a removable refractory wall closing said doorway having an opening therethrough, a refractory tube having a closed end extending through said opening with said closed end extending into said enclosure, said opening having a frusto-conical portion with the widest diameter thereof being located adjacent said chamber and the smallest diameter thereof closely surrounding said tube, the space within said frusto-conical portion between said tube and said one wall being free of obstruction and completely exposed to said enclosure, said frusto-conical portion establishing an area within said tube having a predetermined temperature gradient for a predetermined enclosure temperature, an elongated tray within said tube extending along said frusto-conical portion for supporting articles to be heated at predetermined points along the length thereof, the said space within said tube within said frusto-conical portion being temperature calibrated along the length thereof whereby the temperature at each of said points is known for said predetermined enclosure temperature.

4. A furnace for simultaneously heating articles to a plurality of different known temperatures along a temperature gradient curve comprising refractory walls forming a heating chamber, one of said walls having an opening therethrough, a refractory tube having a closed end extending through said opening with said closed end extending into said enclosure, said opening having a frustoconical portion with the widest diameter thereof being located adjacent said chamber and the smallest diameter thereof closely surrounding said tube, the space within said frusto-conical portion between said tube and said one wall being free of obstruction and completely exposed to said enclosure, said frusto-conical portion establishing an area within said tube having a predetermined temperature gradient for a predetermined enclosure temperature, a movable tray adapted to be extended along said frustoconical portion for supporting articles to be heated at various points along the length thereof, the said space f within said tube within said frusto-conical portion being temperature calibrated along the length thereof whereby the temperature at each of said points is known for said predetermined enclosure temperature.

References Cited by the Examiner UNITED STATES PATENTS Re. 7,644 5/ 77 Armstrong 263-41 463,712 11/91 Ricketts et al. 263-41 2,192,743 3/40 Howe 263-47 2,370,659 3/45 Harris 266-19 X 2,383,209 8/45 Newkrk et al 266-19 CHARLES SUKALO, Primary Examiner. 

2. A FURNACE FOR SIMULTANEOUSLY HEATING A PLURALITY OF ARTICLES TO A PLURALITY OF DIFFERENT KNOWN TEMPERATURES ALONG A TEMPERATURE GRADIENT CURVE COMPRISING REFRACTORY WALLS FORMING A HEATING CHAMBER, ONE OF SAID WALLS HAVING AN OPENING THERETHROUGH, A REFRACTORY TUBE HAVING A CLOSED END EXTENDING THROUGH SAID OPENING WITH SAID CLOSED END EXTENDING INTO SAID ENCLOSURE, SAID OPENING HAVING A FRUSTO-CONICAL PORTION WITH THE WIDEST DIAMETER THEREOF BEING LOCATED ADJACENT SAID CHAMBER AND THE SMALLEST DIAMETER THEREOF CLOSELY SURROUNDING SAID TUBE, THE SPACE WITHIN SAID FRUSTO-CONICAL PORTION BETWEEN SAID TUBE AND SAID ONE WALL BEING FREE OF OBSTRUCTION AND COMPLETELY EXPOSED TO SAID ENCLOSURE, SAID FRUSTO-CONICAL PORTION ESTABLISHING AN AREA WITHIN SAID TUBE HAVING A PREDETERMINED TEMPERATURE GRADIENT FOR A PREDETERMINED ENCLOSURE TEMPERATURE, AN ELONGATED TRAY WITHIN SAID TUBE EXTENDING ALONG SAID FRUSTO-CONICAL PORTION FOR SUPPORTING ARTICLES TO BE HEATED AT PREDETERMINED POINTS ALONG THE LENGTH THEREOF, THE SAID SPACE WITHIN SAID TUBE WITHIN SAID FRUCTOCONICAL PORTION BEING TEMPERATURE CALIBRATED ALONG THE LENGTH THEREOF WHEREBY THE TEMPERATURE AT EACH OF SAID POINTS IS KNOWN FOR SAID PREDETERMINED ENCLOSURE TEMPERATURE. 